1. Field of the Invention
The present invention relates to a ball screw mechanism with a retainer for retaining rolling balls and further to an electric power steering device using the ball screw mechanism.
2. Discussion of the Related Art
Generally, electric power steering devices are provided with an electric motor for assisting a manual steering torque applied to a steering wheel and a ball screw mechanism for converting the rotational motion of the electric motor into the axial motion of a rack shaft. Generally, the ball screw mechanism takes the construction that the rack shaft is provided bodily with a ball screw shaft, that a ball nut is attached to a motor shaft driven by the electric motor, and that the ball screw shaft and the ball nut are screw-engaged through a plurality of rolling balls.
In the electric power steering device of this kind, with rotation of the ball nut, the plurality of rolling balls are rolled spirally (i.e., along the way of screw grooves) between the ball screw shaft and the ball nut in the same direction as they are rotated. Generally, since the rolling balls are arranged closely between the ball screw shaft and the ball nut, rolling motions of the rolling balls bring adjoining rolling balls into collision, at which time, the rotational directions at their contact portions of the adjoining rolling balls become opposite. As a result, there is generated a sliding resistance between the adjoining rolling balls, and the sliding resistance could cause the rotational torque generated by the ball screw mechanism to fluctuate, whereby it is feared that the fluctuation of the rotational torque adversely influences the steering feeling.
In order to solve the aforementioned problem, there has been proposed a ball screw mechanism, in which, as described for example in Japanese examined, published Utility Model Application No. 2-5145, a retainer with elongate holes (retainer grooves) for retaining a plurality of rolling balls to be rollable is arranged between a ball screw shaft and a ball nut. According to the ball screw mechanism described in the utility model application, since the retainer prevents adjoining balls from coming into contact with each other, the fluctuation in rotational torque caused by the aforementioned sliding resistance can be obviated.
However, in the known ball screw mechanism using such a retainer as described in the utility model application, as shown in FIG. 5(A), the groove width of the retainer grooves 2 are formed to be somewhat larger than the diameter of the rolling balls 3 to retain the rolling balls 3 to be rollable, so that each rolling ball 3 is retained in the retainer groove 2 with a play. Thus, as shown in FIG. 5(B), the retainer 1 arranged between the outer circumference 4a of the ball screw shaft 4 and the inner circumference 5a of the ball nut 5 radially deviates (falls) by its dead weight due to the play (clearance) between the retainer grooves 2 and the rolling balls 3. This causes the retainer 1 to come into contact partly with the outer circumference 4a of the ball screw shaft 4 or with the inner circumference 5a of the ball nut 5. Therefore, it is feared that a rubbing noise is generated or that the contact causes abrasion to occur. In order to obviate the foregoing problem, it is required to make larger the radial space between the outer circumference 4a of the ball screw shaft 4 and the inner circumference 5a of the ball nut 5.